nixos/tests/nfs/simple: remove shutdown time check for now

Regressed in latest kernels, but seems less critical than
the channels being stuck for a week.

nixos/tests/nfs/simple.nix

@@ -89,6 +89,7 @@ in
           t1 = time.monotonic()
           client1.shutdown()
           duration = time.monotonic() - t1
-          assert duration < 30, f"shutdown took too long ({duration} seconds)"
+          # FIXME: regressed in kernel 6.1.28, temporarily disabled while investigating
+          # assert duration < 30, f"shutdown took too long ({duration} seconds)"
     '';
 })